Diamond and cubic boron nitride (cBN) particles have found widespread use as superabrasives in a variety of abrading and cutting applications. The worldwide consumption of diamond particles currently exceeds 400 metric tons. Common tools which incorporate superabrasive particles include cutting tools, drill bits, circular saws, grinding wheels, lapping belts, polishing pads, and the like. Among diamond superabrasives, saw diamond has the largest particle size at about 18 to 60 mesh, i.e. 1 mm to 0.25 mm. High quality saw diamonds are generally euhedral having fully grown crystallographic faces. Further, high quality saw diamond ideally have very few defects or inclusions. Standard applications for saw diamonds require high quality diamonds. This is at least partially due to the high impact force encountered during cutting, particularly at high speeds. In contrast, smaller diamond particles, i.e. 60 to 400 mesh or 0.25 mm to 37 μm, such as those used in grinding wheels, create scratches in the surface, which gradually remove material from a workpiece. In such grinding applications, the impact force is typically much less than for cutting applications. Thus, commercially satisfactory smaller diamonds can be produced with less concern for flaws and impurities than is generally acceptable for larger diamonds such as saw diamonds.
Superabrasives are typically formed under ultrahigh pressure, e.g., about 5.5 GPa and high temperature, e.g., 1300° C. Under ultrahigh pressure conditions during crystal growth, the pressure tends to continually decay due to the volume contraction associated with diamond formation. Further, temperatures within the growth regions can increase due to increases in electrical resistance associated with the diamond formation. Hence, it is very difficult to maintain optimal conditions of pressure and temperature for homogeneous growth of diamond grits. Saw diamond grits are typically grown under ultrahigh pressure over a much longer time, e.g., 40 minutes than that required to grow smaller grinding grits, e.g., about 1 minute. Consequently, saw diamond grits are very difficult to grow, particularly those having high quality. Saw grits with high impact strength are characterized by a euhedral crystal shape and very low inclusions of either metal or graphite. Hence, very tight controls of pressure and temperature are required over extended periods of time to produce high quality diamonds. These difficulties partially account for the abundance of companies which can grow saw grits, while very few companies are capable of growing high grade saw grits having larger sizes.
Typical methods for synthesizing larger high quality diamonds involve ensuring uniformity of raw materials such as graphite and metal catalyst and carefully controlling process temperature and pressures. High pressure high temperature (HPHT) processes used in diamond growth can employ reaction volumes of over 200 cm3. Most often, the graphite to diamond conversion in the reaction volume can be up to about 30%. Unfortunately, typical processes also result in the crystals having external flaws, e.g., rough surfaces, and undesirable inclusions, e.g., metal and carbon inclusions. Therefore, increased costs are incurred in segregating acceptable high strength diamonds from weaker, poor quality diamonds.
One major factor to consider in diamond synthesis of high grade saw diamonds is providing conditions such that nucleation of diamond occurs uniformly and nearly simultaneously. Random nucleation methods typically allow some regions of raw materials to be wasted while other regions are densely packed with diamond crystals having a high percentage of defects. Some diamond synthesis methods have improved nucleation uniformity somewhat; however, during diamond growth local changes in pressure can occur. If heating is accomplished by passing electrical current directly through the reaction cell, then diamond growth can also interfere with the electrical current used to control heating. The results of such interference are non-uniformities and fluctuations in the temperature and pressure gradients across the reaction cell and thus a wide distribution of crystal sizes, crystal shapes, and inclusion levels. Despite these difficulties, by providing highly homogeneous starting materials and carefully controlling process conditions, the volume efficiency of the reaction cell is still typically less than desired. This marginal yield still wastes large amounts of raw materials, reduces production efficiencies, and leaves considerable room for improvement.
Other methods for synthesizing large industrial diamond particles include forming layers of solid disks of graphite and/or solid disks of catalyst. Diamond nucleation then occurs at the interface between graphite and catalyst layers. The firing temperature for graphite rods that are cut into disks can vary from region to region, thus affecting the microstructure and composition of the disk. Further, during mechanical formation of graphite into a rod, the graphite microstructure can change, e.g., the outer regions exhibit a skin effect during extrusion. As a result, graphite disks tend to have regions which vary in porosity, degree of graphitization, ash content, and the like. Diamonds grown under such conditions tend to nucleate at different times and experience varying growth rates, thus producing diamonds having a wide size distribution and increasing the number of flawed diamonds due to intergrowth, overgrowth, i.e. fast growth rates, and uneven growth, i.e. asymmetric growth.
Recently, efforts have been made in using patterned placement of diamond seeds. However, diamond growth on the defined nucleation sites is greatly determined and limited by the materials surrounding the seeds. Diamond or other superabrasive particle growth goes through phases in the growth process. The materials needed at the beginning of the growth process are different from those necessary towards the end of processing. However, the growth conditions do not permit changing or altering the materials in-process. Without supplying the seeds with the proper materials at the proper stages in growth, the size and quality of synthetic diamonds will necessarily be limited.